Actuator for a heart assist device

ABSTRACT

An actuator ( 10 ) for a heart assist device. The actuator ( 10 ) includes an inflatable balloon ( 12 ) and a shroud or wrap ( 14 ). The inflatable balloon ( 12 ) has a first body portion ( 22 ), a second body portion ( 26 ) and a flexure region joining ( 24 ) the first ( 22 ) and second ( 26 ) body portions. The shroud or wrap ( 14 ) is positioned adjacent the first body portion ( 24 ) and has a peripheral extent at least equal to the peripheral extent of the balloon flexure region ( 24 ). The balloon ( 12 ) and the shroud or wrap ( 14 ) are shaped such that the shroud or wrap ( 14 ) restrains a part of the balloon first body portion ( 22 ) at or near the flexure region ( 24 ) against displacement towards the shroud or wrap (outward displacement) past a predetermined limit but allows unrestrained displacement away from the shroud or wrap (inward displacement).

FIELD OF THE INVENTION

The present invention relates generally to heart assist devices, systemsand methods and, more particularly, to an actuator for acounter-pulsation heart assist device.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,630,597 and International PCT Patent Application No.PCT/US00/22992 (WO 01/13974) both disclose heart assist devices thatutilise an inflatable balloon that is positioned within an arterialvessel of a patient. The balloons replace a resected portion of thevessel and are cyclically inflated and deflated to expand into thevessel and thus assist in blood displacement during diastole and retractfrom within the vessel during systole.

Both of the above documents disclose devices that have a relativelyrigid shell with an inlet/outlet port which is connected to a fluidpump. The flexible balloon seals around the periphery of the shell andextends back over the shell for some or all of its exterior. Anotherlayer of material is then placed over the balloon exterior, adjacent theshell, to secure the balloon to the shell.

The Applicant's International PCT Patent Application No's.PCT/AU00/00654, PCT/AU01/01187, and PCT/AU02/00974 all discloseactuators that were found to fail due to fatigue in the ballooninterfacing with the aorta.

It is an object of the present invention to provide an improvedactuator.

SUMMARY OF THE INVENTION

Accordingly, in a first aspect, the present invention provides anactuator for a heart assist device, the actuator including:

an inflatable balloon having a first body portion, a second body portionand a flexure region joining the first and second body portions; and

a shroud or wrap adjacent the first body portion and having a peripheralextent at least equal to the peripheral extent of the balloon flexureregion;

wherein the balloon and the shroud or wrap are shaped such that theshroud or wrap restrains a part of the balloon first body portion at ornear the flexure region against displacement towards the shroud or wrap(outward displacement) past a predetermined limit but allowsunrestrained displacement away from the shroud or wrap (inwarddisplacement).

The balloon and the shroud are preferably shaped such that the shroudrestrains said part of the balloon first body portion at or near theflexure region against outward displacement during inflation of theballoon but allows unrestrained inward displacement during deflation.

Preferably, during inward displacement, at least part of the innersurface of the balloon second body portion is able to be drawn againstat least part of the inner surface of the balloon first body portion.

The shroud is preferably generally inwardly concave, most preferablyelongated, and elliptical.

The first body portion, second body portion and flexure region arepreferably integrally formed, most preferably by dip moulding.

The actuator preferably also includes a bushing adapted for connectionto a motive power source.

The balloon preferably also includes a neck portion joined to the firstportion, the neck portion being adapted for sealing connection with thebushing.

The shroud preferably also includes a neck portion adapted for sealingconnection with the balloon neck portion.

In a second aspect, the present invention provides an actuator for aheart assist device, the actuator including:

a bushing adapted for connection to a hydraulic or pneumatic powersource; and

an inflatable balloon having a narrower neck portion adapted for sealingconnection with the bushing exterior, wider first and second bodyportions and an arcuate flexure region joining the first and second bodyportions, the first body portion having a first end adjacent the neckportion and a second end adjacent the second body portion and beinggenerally inwardly concave, the second body portion being inwardlyconcave when the balloon is inflated and generally outwardly concavewhen the balloon is deflated.

The device preferably also includes a shroud or wrap having a bodyportion with a peripheral extent at least equal to the peripheral extentof the balloon first and second body portions.

The balloon and the shroud or wrap are preferably shaped such that apart of the balloon first body portion at or near the flexure region isrestrained against outward displacement past a predetermined limit bythe shroud or wrap but unrestrained against inward displacement.

Preferably, during inward displacement, at least part of the innersurface of the balloon second body portion is able to be drawn directlyagainst at least part of the inner surface of the balloon first bodyportion.

In a third aspect, the present invention provides a heart assist deviceincluding:

a hydraulic or pneumatic power source; and

an actuator including:

a bushing adapted for operative connection to the motive power source;

an inflatable balloon having a narrower neck portion adapted for sealingconnection with the bushing exterior, wider first and second bodyportions and a flexure region joining the first and second bodyportions, the first body portion having a first end adjacent the neckportion and a second end adjacent the second body portion and beinggenerally inwardly concave, the second body portion being generallyinwardly concave when the balloon is inflated and generally outwardlyconcave when the balloon is deflated; and

a shroud or wrap having a body portion with a peripheral extent at leastequal to the peripheral extent of the balloon first and second bodyportions,

wherein the balloon and the shroud are shaped such that a part of theballoon first body portion at or near the flexure region is restrainedagainst outward displacement by the shroud past a predetermined limitbut unrestrained against inward displacement.

Preferably, during inward displacement, at least part of the innersurface of the balloon second body portion is able to be drawn againstat least part of the inner surface of the balloon first body portion.

Preferably, the balloon and shroud are shaped such that substantiallyall of the balloon first body portion is restrained against outwarddisplacement by the shroud and unrestrained against inward displacement.

In its preferred form, the heart assist device is configured forextra-aortic counter-pulsation. In this form, the balloon is positionedon the exterior of an arterial vessel.

In a further form, the heart assist device is configured for use eitheras an interposition graft in which the device replaces a completelyresected section of the aorta or as an aortic patch in which an apertureis formed in the aorta which is filled with the device.

When the balloon is inflated, the flexure region preferably has a radiusof curvature of at least 0.1 mm, more preferably a radius of curvatureof approximately 1.0 mm and most preferably a radius of curvature ofapproximately 3.0 mm.

The ratio of the diameter of the balloon neck portion to the balloonflexure region is preferably no more than approximately 4:1, and morepreferably approximately 3:1 and most preferably approximately 2:1.

The bushing preferably has an inlet/outlet bore. The bore preferablyalso includes one or more internal restrictions adapted to preventsuction of the balloon into the bore.

The balloon is preferably formed from silicone, polyurethane or apolyurethane-polysiloxane block co-polymer. The balloon is preferablyformed by mandrel dipping. The balloon is preferably formed by dipping asuitably shaped mandrel into the polymer and allowing a thin coating ofthe polymer to cure on the mandrel. The balloon is preferably made of 2to 4 coatings, with a total thickness of 150-300 microns. The ballooncan then be removed from the mandrel.

The balloon neck portion is preferably a snug sealing fit over thebushing exterior. The shroud or wrap preferably has a neck portion thatis a snug sealing fit over the balloon neck portion.

The bushing preferably has a slightly tapered neck portion adapted forengagement with the balloon neck portion. The bushing neck portionpreferably has a converging taper in the direction of the balloon.

The balloon is preferably held in place on the aorta by a flexible wrapwhich extends about the aorta and bears against the first body portionof the balloon or a shroud mounted thereon. The wrap is preferablyshaped to fit the second body portion of the balloon, and if desiredalso the neck portion. The wrap is preferably inelastic or slightlyelastic so that its stretch and flexibility characteristicssubstantially match those of the native aorta.

In a fourth aspect, the present invention provides a flexible inflatableballoon for a blood displacing heart assist device, the balloonincluding:

a neck portion having first and second ends;

a substantially annular first body portion connected at its innerperiphery to the neck portion second end; and

a substantially oval or circular second body portion connected at itsouter periphery to the outer periphery of the first body portion,

the outer peripheries of the first and second body portions areconnected along an annular inwardly concavely curved flexure portionadapted to maintain a radius of curvature during movement of the secondbody portion between inwardly concave and outwardly concave duringdeflation and inflation of the balloon respectively.

The balloon is advantageously formed as a single piece. This avoids thepresence of a seam line as is disclosed in U.S. Pat. No. 4,630,597 orInternational patent specification WO 01/13974. Such seam lines havebeen found by the present inventors to raise stress levels in theballoon and reduce the operational life of the balloon.

A shroud can be provided to overlie and abut the annular first bodyportion of the balloon. The shroud is preferably shaped such that theshroud restrains said portion of the balloon at or near the flexureregion against outward displacement during inflation of the balloon butallows unrestrained inward displacement during deflation. The shroud ispreferably generally inwardly concave, most preferably elongated, andelliptical. The shroud preferably also includes a neck portion adaptedfor sealing connection with the balloon neck portion. The shroud may actto facilitate bonding a wrap to the first body portion of the balloon.

The balloon preferably also includes a bushing adapted for connection toa hydraulic or pneumatic power source. The bushing also acts to preventinward collapse of the balloon neck portion during deflation. If desiredthe bushing can be formed with internal restrictions such as flutes,ribs or secondary lumens to prevent the balloon being sucked into thebushing during deflation of the balloon. The neck portion of the balloonis preferably adapted for sealing connection with the bushing. Thebushing preferably has a taper adapting the relatively large diameter ofthe neck of the balloon to the relatively small diameter of a hydraulicor pneumatic fluid line connecting the balloon to a power source. Thistaper is preferably elongated to enhance the flexibility of the bushingalong its central axis.

In a fifth aspect, the present invention provides an actuator for aheart assist device, the actuator including:

a flexible inflatable balloon having a neck portion connected at one endto a bulbous body portion having a first side and a second side; and

a substantially inelastic shroud or wrap having a flared portion thatextends over the adjacent first side of the balloon bulbous portion,

wherein, during deflation, the second side of the bulbous body portionis able to be drawn against the first side of the bulbous body portion.

The shroud or wrap preferably supports the first side of the balloonbulbous portion against substantial movement whilst the second side ofthe balloon bulbous portion is free to move during inflation anddeflation.

In a sixth aspect, the present invention provides a method of providingextra-aortic heart assistance using the actuator of the first or thesecond or the fifth aspect or the heart assist device according to thethird aspect or the balloon of the fourth aspect, the method includingmounting the balloon second body portion adjacent the exterior of anarterial vessel.

In a seventh aspect, the present invention provides a method ofproviding intra-aortic heart assistance using the actuator according tothe first or the second or the fifth aspect or the heart assist deviceaccording to the third aspect, the method including resecting a portionof an arterial vessel and mounting the balloon with the balloon secondbody portion sealingly replacing the resected arterial portion.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, by way ofexamples only, with reference to the accompanying drawings in which:

FIG. 1 is an exploded perspective view of a first embodiment of anactuator according to the invention;

FIG. 2 is an assembled, partial cut away, perspective view of theactuator shown in FIG. 1;

FIG. 3 is an assembled, cross sectional view of the actuator shown inFIG. 1 along the line 3-3;

FIG. 4 is an assembled, cross sectional view of the actuator shown inFIG. 1 along the line 4-4;

FIG. 5 is a cross sectional view of the second embodiment of an actuatoraccording to the invention along the line 5-5;

FIG. 6 is an underside view of the bushing used in the actuator shown inFIG. 5;

FIG. 7 is a cut away perspective view of a balloon for a thirdembodiment of an actuator according to the invention;

FIG. 8 is a top view of the balloon shown in FIG. 7;

FIG. 9 is a cut away side view of a fourth embodiment of an actuatoraccording to the invention;

FIG. 10 is a perspective view of the actuator shown in FIG. 9; and

FIG. 11 is a front view of the actuator shown in FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an exploded perspective view of an actuator 10 according toa first embodiment of the present invention, which is sized forpaediatric use. The actuator 10 has a flexible, inflatable balloon 12,an inelastic shroud 14, and a relatively rigid bushing 16.

The balloon 12 is formed from Polyurethane or similar biocompatible andbiostable material by mandrel dipping. The balloon 12 has a narrowerneck portion 18 which is connected by a flared part 20 to a wider firstbody portion 22. A flexure region 24, which extends generally around theexterior of the balloon 12 at its widest part connects a second bodyportion 26 (see FIGS. 2 to 4) to the first body portion 22.

When viewed in plan, the first and second body portions are generallyelliptical in shape and have a maximum width and length of about 20-35mm and 50-90 mm respectively for adult sized balloons. The balloon neckportion has a diameter of approximately 10-14 mm at its distal end 18 aand, at its proximal end 18 b, it is elliptical in shape with dimensionsof approximately 30-50 mm in the long axis and 15-30 mm in the shortaxis of the balloon 12, and with a converging taper therebetween.Paediatric balloons are also considered in this same application, butscaled downward appropriately.

The shroud 14 is formed, by mandrel dipping, from a materialsubstantially equivalent to the material used for the balloon, such as aPolyurethane or a Polyurethane-Polysiloxane block co-polymer. The shroud14 has a narrower neck portion 28 which is joined to a wider bodyportion 30 by a flared part 32. The shroud body portion 30 is alsogenerally elliptical in shape when viewed in plan and has a largerperipheral extent (ie extends further in all directions) than theballoon body portions 22 and 26 and flexure region 24. Moreparticularly, the shroud body portion 30 has a length (along the longaxis of the balloon 12) of 60-100 mm and a minimum width of (across theshort axis of the balloon 12) of 20 mm. The shroud neck portion 28 issized to be a snug sealing fit over the exterior of the balloon neckportion 18.

The bushing 16 is formed, by injection moulding, from a flexible plasticsuch as Polyurethane or a similar material. The bushing 16 has a hollowbore 36 which is adapted for sealing connection with a fluid line from amotive power source such as a fluid pump (not shown). Suitable pumps aredisclosed in the Applicant's International PCT Patent Application No.PCT/AU02/00974 entitled “A fluid pressure generating means”, thecontents of which are hereby incorporated by cross reference. Thebushing 16 also has a relatively more tapered distal part 38 and arelatively less tapered proximal part 40. The proximal part 40 is sizedto be a snug sealing fit within the interior of the balloon neck portion18.

FIG. 2 shows the device 10 after assembly. The assembly comprisesinitially stretching the neck portion 18 of the balloon 12 so that theproximal part 40 of the bushing 16 can be inserted therein. The balloon12 is retained adjacent the bushing 16 by the shroud 14 being forcedpast the distal part 38 of the bushing 16 until it is a snug sealing fiton the exterior of the balloon neck portion 18 as shown. The bushingproximal part 40, the balloon neck portion 18, and the shroud neckportion 28 all have a common taper angle. The components are also bondedtogether with a suitable adhesive in order to ensure an effective sealtherebetween.

The operation of the device 10 will now be described by with referenceto FIGS. 3 and 4. FIG. 3 is a cross-sectional (anterior) view along thelongitudinal direction of the aorta. FIG. 4 is a partial cross-sectionalview orientated at 90 degrees from that of FIG. 3.

For extra-aortic heart assistance, the device 10 is placed with theballoon second portion 26 adjacent the exterior of an arterial vessel,most preferably the outer part of the ascending aorta (not shown). Aflexible, relatively inelastic wrap is placed over the shroud 30 andaround the aorta in order to retain the device 10 in place. Wraps arewell known in the art and thus will not be described in further detail.The wrap can also be used in place of the shroud.

In use, fluid is cyclically driven to and from the balloon 12, via thebushing bore 36, to cyclically inflate and deflate the balloon 12. Theinflated balloon 12 is shown in solid line in FIG. 3. The inflatedballoon 12 compresses the aorta and thus assists in blood displacementduring diastole. When the balloon 12 is deflated it retracts to theposition shown in phantom line, which allows the aorta to return to itsnatural shape during systole.

As FIGS. 3 and 4 show, when the balloon 12 is inflated the shroud 14restrains the balloon first portion 22, up to and including the partadjacent the flexure region 24, against outward displacement past apredetermined limit. That limit being defined by the shape of the shroudbody portion 30. However, the shroud 14 does not restrain the inwarddisplacement of the balloon first portion 22, particularly that part ator near the flexure region 24, during deflation. This allows the balloon12 to always retain a relatively large radius of curvature (e.g. 1.0 mm)adjacent the flexure region 24, which is the part of the balloon 12 thatundergoes the most deformation. This advantageously minimises stress andstrain concentration in the flexure region 24, which results in a muchmore reliable and longer lasting balloon 18.

This is in contrast to the balloons shown in the two prior art documentsmentioned previously which are restrained at a region equivalent to theflexure region for both inward and outward displacement. These balloonsundergo a movement akin to pivoting or bending at their free edge. Thisresults in high levels of stress concentration, and associated higherrisk of failure, in those balloons.

Further, cycle testing of a group of actuators configured for use withsheep (and corresponding to the actuator 10 described above) wereterminated after the equivalent of two years cycling without anyfailures. In addition, cycle testing of similar actuators configured foruse with humans has achieved the equivalent of 18 months use without anyfailures.

The actuator 10 is also simpler, and thus cheaper, to assemble andmanufacture than the prior art devices as the bushing 16 only engageswith the balloon 12 in the region of their respective neck portions 40,18, as opposed to positioning the bushing (or shell) more fully withinthe body portions of the balloon.

It should also be noted that the actuator 10 is designed to be appliedto the outside of a blood vessel and is thus advantageously non-bloodcontacting as compared to the prior art devices discussed in theBackground of the Invention, which are positioned within the wall of ablood vessel.

It should also be noted that the inward concavity of the balloon (whendeflated) is designed to fit the arcuate ascending aorta particularly,to allow a conformal wrapping of the assembled actuator around theascending aorta With balloon inflation, the aortic wall is displaced ina “thumbprinting” manner, which has been shown by finite elementanalysis to cause minimal strain concentration in the aortic wall andalso to provide maximal blood volume displacement.

In a more preferred embodiment, the balloon has in its longitudinalplane, a gentle arc of the order of radius of 150-300 mm, to accommodatethe slight spiral nature of the ascending aorta, to allow furtherconformal fitting of the assembled device.

FIGS. 5 and 6 show a second embodiment of actuator 50 according to theinvention. The device 50 is similar to the first embodiment and likereference numerals are used to indicate like features.

The major difference between the two embodiments is that the bushing 16in the device 50 includes a series of internal projections 52 whichserve to stop the balloon 18 being sucked into the bore 36 of thebushing during large amounts of suction/deflation. Such large amounts ofsuction/deflation can occur during the calibration cycle of some of thepumps suitable for use with the blood displacing device 50.

FIGS. 7 and 8 show a balloon 60 for a third embodiment of actuatoraccording to the invention. The balloon 60 is sized for adult use is butis otherwise similar to the first embodiment and like reference numeralsare used to indicate like features. When viewed in plan, the first andsecond body portions 24 and 26 are generally elliptical in shape andhave a maximum width and length of 35 mm and 80 mm respectively. Theballoon neck portion has a diameter of approximately 10 mm at its distalend 18 a and approximately 26-44 mm (oval in cross-section) at itsproximal end 18 b and a conical converging taper therebetween.

FIGS. 8 to 10 show a fourth embodiment of actuator 70 according to theinvention. Like features to earlier embodiments are indicated with likereference numerals. The actuator 70 utilises the balloon 60 shown inFIGS. 7 and 8. The bushing 16 of the actuator 70 differs from earlierembodiments in that it is substantially hollow with a centralcylindrical part 16 a attached at one end to an outer flared conicalpart 16 b. A series of radial webs 16 c are provided between the centralcylindrical part 16 a and the flared conical part 16 b. A flexible,relatively inelastic wrap 72 is provided over the balloon 60, which hasan opening 72 a through which the bushing 16 protrudes.

It will be appreciated by the persons skilled in the art that numerousvariations and/or modifications can be made to the invention as shown inthe specific embodiment without departing from the spirit or scope ofthe invention as broadly defined. For example, the blood displacingdevices are described above in relation to extra-aorticcounter-pulsation but also suitable for intra aortic counter-pulsation.In the latter the second portion of the balloon replaces a resectedportion of arterial vessel, with the opening made in the resectedarterial vessel being sealingly connected to the balloon adjacent theflexure region.

1. Actuator for a heart assist device, the actuator including: aninflatable balloon having a first body portion, a second body portionand a flexure region joining the first and second body portions; and ashroud or wrap adjacent the first body portion and having a peripheralextent at least equal to the peripheral extent of the balloon flexureregion; wherein the balloon and the shroud or wrap are shaped such thatthe shroud or wrap restrains a part of the balloon first body portion ator near the flexure region against displacement towards the shroud orwrap past a predetermined limit but allows unrestrained displacementaway from the shroud or wrap.
 2. The actuator as claimed in claim 1,wherein the balloon and the shroud are shaped such that the shroudrestrains said part of the balloon first body portion at or near theflexure region against outward displacement during inflation of theballoon but allows unrestrained inward displacement during deflation. 3.The actuator as claimed in claim 1, wherein, during inward displacement,at least part of the inner surface of the balloon second body portion isable to be drawn against at least part of the inner surface of theballoon first body portion.
 4. The actuator as claimed in claim 1,wherein the shroud is generally inwardly concave.
 5. The actuator asclaimed in claim 4, wherein the shroud is elongated and elliptical. 6.The actuator as claimed in claim 1, wherein the first body portion,second body portion and flexure region are integrally formed.
 7. Theactuator as claimed in claim 6, wherein the first body portion, secondbody portion and flexure region are integrally formed by dip moulding.8. The actuator as claimed in claim 1, wherein the actuator alsoincludes a bushing adapted for connection to a motive power source. 9.The actuator as claimed in claim 8, wherein the balloon also includes aneck portion joined to the first portion, the neck portion being adaptedfor sealing connection with the bushing.
 10. The actuator as claimed inclaim 9, wherein the shroud also includes a neck portion adapted forsealing connection with the balloon neck portion.
 11. An actuator for aheart assist device, the actuator including: a bushing adapted forconnection to a hydraulic or pneumatic power source; and an inflatableballoon having a narrower neck portion adapted for sealing connectionwith the bushing exterior, wider first and second body portions and anarcuate flexure region joining the first and second body portions, thefirst body portion having a first end adjacent the neck portion and asecond end adjacent the second body portion and being generally inwardlyconcave, the second body portion being inwardly concave when the balloonis inflated and generally outwardly concave when the balloon isdeflated.
 12. The actuator as claimed in claim 11, wherein the devicealso includes a shroud or wrap having a body portion with a peripheralextent at least equal to the peripheral extent of the balloon first andsecond body portions.
 13. The actuator as claimed in claim 12, whereinthe balloon and the shroud or wrap are shaped such that a part of theballoon first body portion at or near the flexure region is restrainedagainst outward displacement past a predetermined limit by the shroud orwrap but unrestrained against inward displacement.
 14. The actuator asclaimed in claim 13, wherein during inward displacement, at least partof the inner surface of the balloon second body portion is able to bedrawn directly against at least part of the inner surface of the balloonfirst body portion.
 15. A heart assist device including: a hydraulic orpneumatic power source; and an actuator including: a bushing adapted foroperative connection to the motive power source; an inflatable balloonhaving a narrower neck portion adapted for sealing with the bushingexterior, wider first and second body portions and a flexure regionjoining the first and second body portions, the first body portionhaving a first end adjacent the neck portion and a second end adjacentthe second body portion and being generally inwardly concave, the secondbody portion being generally inwardly concave when the balloon isinflated and generally outwardly concave when the balloon is deflated;and a shroud or wrap having a body portion with a peripheral extent atleast equal to the peripheral extent of the balloon first and secondbody portions, wherein the balloon and the shroud are shaped such that apart of the balloon first body portion at or near the flexure region isrestrained against outward displacement by the shroud past apredetermined limit but unrestrained against inward displacement. 16.The device as claimed in claim 15, wherein, during inward displacement,at least part of the inner surface of the balloon second body portion isable to be drawn against at least part of the inner surface of theballoon first body portion.
 17. The device as claimed in claim 15,wherein the balloon and shroud are shaped such that substantially all ofthe balloon first body portion is restrained against outwarddisplacement by the shroud and unrestrained against inward displacement.18. The device as claimed in claim 15, wherein the heart assist deviceis configured for extra-aortic counter-pulsation and the balloon ispositioned on the exterior of an arterial vessel.
 19. The device asclaimed in claim 15, wherein the heart assist device is configured foruse as an interposition graft in which the device replaces a completelyresected section of the aorta.
 20. The device as claimed in claim 15,wherein the heart assist device is configured for use as an aortic patchin which an aperture is formed in the aorta which is filled with theheart assist device.
 21. The device as claimed in claim 15, wherein,when the balloon is inflated, the flexure region has a radius ofcurvature of at least 0.1 mm.
 22. The device as claimed in claim 21wherein, when the balloon is inflated, the flexure region has a radiusof curvature of approximately 1.0 mm.
 23. The device as claimed in claim22, wherein, when the balloon is inflated, the flexure region has aradius of curvature of approximately 3.0 mm.
 24. The device as claimedin claim 15, wherein the ratio of the diameter of the balloon neckportion to the balloon flexure region is no more than approximately 4:1.25. The device as claimed in claim 15, wherein the ratio of the diameterof the balloon neck portion to the balloon flexure region isapproximately 3:1.
 26. The device as claimed in claim 15, wherein theratio of the diameter of the balloon neck portion to the balloon flexureregion is approximately 2:1.
 27. The device as claimed in claim 15,wherein the bushing has an inlet/outlet bore.
 28. The device as claimedin claim 27, wherein the bore also includes one or more internalrestrictions adapted to prevent suction of the balloon into the bore.29. The device as claimed in claim 15, wherein the balloon is formedfrom silicone, polyurethane or a polyurethane-polysiloxane blockco-polymer.
 30. The device as claimed in claim 15, wherein the balloonis formed by mandrel dipping.
 31. The device as claimed in claim 15,wherein the balloon is formed by dipping a suitably shaped mandrel intothe polymer and allowing a thin coating of the polymer to cure on themandrel.
 32. The device as claimed in claim 31, wherein the balloon ismade of 2 to 4 coatings of the polymer.
 33. The device as claimed inclaim 32, wherein the balloon a total thickness of 150-300 microns. 34.The device as claimed in claim 15, wherein the balloon neck portion is asnug sealing fit over the bushing exterior.
 35. The device as claimed inclaim 34, wherein the shroud or wrap has a neck portion that is a snugsealing fit over the balloon neck portion.
 36. The device as claimed inclaim 35, wherein the bushing has a slightly tapered neck portionadapted for engagement with the balloon neck portion.
 37. The device asclaimed in claim 36, wherein the bushing neck portion has a convergingtaper in the direction of the balloon.
 38. The device as claimed inclaim 15, wherein the balloon is held in place on the aorta by aflexible wrap which extends about the aorta and bears against the firstbody portion of the balloon or a shroud mounted thereon.
 39. The deviceas claimed in claim 38, wherein the flexible wrap is shaped to fit thesecond body portion of the balloon.
 40. The device as claimed in claim39, wherein the flexible wrap is also shaped to fit the neck portion.41. The device as claimed in claim 38, wherein the flexible wrap isinelastic or slightly elastic so that its stretch and flexibilitycharacteristics substantially match those of the native aorta.
 42. Aflexible inflatable balloon for a blood displacing heart assist device,the balloon including: a neck portion having first and second ends; asubstantially annular first body portion connected at its innerperiphery to the neck portion second end; and a substantially oval orcircular second body portion connected at its outer periphery to theouter periphery of the first body portion, the outer peripheries of thefirst and second body portions are connected along an annular inwardlyconcavely curved flexure portion adapted to maintain a radius ofcurvature during movement of the second body portion between inwardlyconcave and outwardly concave during deflation and inflation of theballoon respectively.
 43. The balloon as claimed in claim 42, whereinthe balloon is formed as a single piece.
 44. The balloon as claimed inclaim 42, and further including a shroud adapted to overlie and abut theannular first body portion of the balloon.
 45. The balloon as claimed inclaim 44, wherein the shroud is shaped such that the shroud restrainssaid portion of the balloon at or near the flexure region againstoutward displacement during inflation of the balloon but allowsunrestrained inward displacement during deflation.
 46. The balloon asclaimed in claim 44, wherein the shroud is generally inwardly concave.47. The balloon as claimed in claim 46, wherein the shroud is elongated,and elliptical.
 48. The balloon as claimed in claim 44, wherein theshroud also includes a neck portion adapted for sealing connection withthe balloon neck portion.
 49. The balloon as claimed in claim 48,wherein the shroud is adapted to facilitate bonding of a wrap to thefirst body portion of the balloon.
 50. The balloon as claimed in claim42, and further including a bushing adapted for connection to ahydraulic or pneumatic power source.
 51. The balloon as claimed in claim50, wherein the bushing is formed with internal restrictions such asflutes, ribs, or secondary lumens to prevent the balloon being suckedinto the bushing during deflation of the balloon.
 52. The balloon asclaimed in claim 50, wherein the neck portion of the balloon is adaptedfor sealing connection with the bushing.
 53. The balloon as claimed inclaim 50, wherein the bushing has a taper adapting the relatively largediameter of the neck of the balloon to the relatively small diameter ofa hydraulic or pneumatic fluid line connecting the balloon to a powersource.
 54. The balloon as claimed in claim 53, wherein this taper iselongated to enhance the flexibility of the bushing along its centralaxis.
 55. The balloon as claimed in claim 42, wherein the balloon has inits longitudinal plane, a gentle arc of the order of radius of 150-300mm.
 56. An actuator for a heart assist device, the actuator including: aflexible inflatable balloon having a neck portion connected at one endto a bulbous body portion having a first side and a second side; and asubstantially inelastic shroud or wrap having a flared portion thatextends over the adjacent first side of the balloon bulbous portion,wherein, during deflation, the second side of the bulbous body portionis able to be drawn against the first side of the bulbous body portion.57. The actuator as claimed in claim 56, wherein the shroud or wrapsupports the first side of the balloon bulbous portion againstsubstantial movement whilst the second side of the balloon bulbousportion is free to move during inflation and deflation.
 58. A method ofproviding extra-aortic heart assistance using the actuator claimed inclaim 1, the method including mounting the balloon second body portionadjacent the exterior of an arterial vessel.
 59. A method of providingintra-aortic heart assistance using the actuator claimed in claim 1, themethod including resecting a portion of an arterial vessel and mountingthe balloon with the balloon second body portion sealingly replacing theresected arterial portion.
 60. A method of providing extra-aortic heartassistance using the heart assist device claimed in claim 15, the methodincluding mounting the balloon second body portion adjacent the exteriorof an arterial vessel.
 61. A method of providing extra-aortic heartassistance using the balloon claimed in claim 40, the method includingmounting the balloon second body portion adjacent the exterior of anarterial vessel.
 62. A method of providing intra-aortic heart assistanceusing the heart assist device claimed in claim 15, the method includingresecting a portion of an arterial vessel and mounting the balloon withthe balloon second body portion sealingly replacing the resectedarterial portion.